The invention relates to an optical system, and more particularly to an off-axis liquid crystal projector using a single chip.
Reflective liquid crystal display (LCD) panels have many advantages over conventional transmissive LCD panels in many aspects. As a result, there has been an increasing trend to adopt reflective LCD panels in liquid crystal projectors. For example, the aperture ratio (i.e. the ratio of area between the actual size area of a pixel and the area of that pixel that can transmit light) of a transmissive LCD panel is limited, and it is necessary to add isolating material between pixels, which can generate pixilated images. For transmissive LCD panels, it is difficult to enable display devices having high resolution and high brightness at a reasonable production cost. Reflective LCD panels achieve desired aperture ratio and adopt specular material characterized by high reflectivity. Consequently, the reflective LCD panel addresses many of the problems of the transmissive LCD panel.
FIG. 1 shows a conventional on-axis reflective projection system 100 comprising a light source 102, a polarizing beam splitter (PBS) 104, a color separator 106, a plurality of LCD panels 108, such as red, blue and green panels, and a projection lens 110. The light source 102 emits white light to the PBS 104. The PBS 104 only allows light of certain polarization therethrough and reflects light of other polarization to the color separator 106, according to a system axis 112. The color separator 106 then separates the red, blue and green components in the light and allows them to progress toward the respective red, blue and green LCD panels 108. Each LCD panel 108 is controlled by a system, such as a computer or other image source (not shown) and the reflected light from the pixels is selectively modulated to generate a light forming color image which is then reflected to the color separator 106. The color separator 106 combines incident red, green and blue light into a whole-color light and outputs it to the PBS 104 along system axis 112. PBS 104 allows only modulated light to pass to the projection lens 110, whereby the light is focused and projected onto a screen (not shown).
Another on-axis reflective projection system adopts a single chip having color filters. This type of on-axis single chip projector requires no color separator or combining unit for the single chip having color filters, but there is still a need for PBS. The on-axis single chip projector, however, still has many unsolved problems. Firstly, light reflection efficiency is reduced by the single chip having color filters and the PBS in the light path. Secondly, if the incident angle of lights to the PBS increases, there is a serious drop in transmissivity. Thirdly, skew ray caused by the PBS also reduces contrast ratio (CR).